A dispensing circuit of the type indicated above is known from U.S. Pat. No. 6,457,607 which describes a pumping unit comprising a stepper motor which operates a bellows pumping chamber for sending a fluid product to a dispensing duct along which a non-return valve is positioned. In order to dispense a desired quantity of fluid product, the stepper motor is operated for a determined number of steps which bring about a corresponding reduction in volume of the pumping chamber and therefore the delivery of a specific volume of fluid product.
The applicant has found that, under particular conditions of operation or for particular fluid products, the volume of fluid product delivered by the dispensing circuit may not correspond to the actual volume of fluid product desired. The applicant has found that sometimes the fluid product to be delivered, which is substantially incompressible per se, may incorporate a quantity of highly compressible air. During the operation of the above-mentioned circuit, therefore, the reduction in the volume of the pumping chamber caused by the operation of the motor is in part wasted in compressing the air inside the bellows and the dispensing duct, without a corresponding effective delivery of the fluid product. Consequently, the volume of fluid product actually delivered by the above-mentioned dispensing circuit may be smaller than that expected and desired on the basis of the operation of the motor.
The traditional solution to the problem indicated above is to calibrate the dispensing circuit in such a manner as to determine correcting values for the activation period or activation stroke of the motor, which are such as to compensate for the effect caused by the presence of air in the circuit and inside the incompressible fluid product to be delivered. It should, however, be noted that the quantity of air incorporated inside the incompressible fluid to be delivered is not constant over time and may vary within rather wide limits, to such an extent as to influence the precision and repeatability performance in the delivery operations of the above-mentioned known circuit. In particular, the applicant has found that often the quantity of air incorporated inside the colouring agent is at a maximum immediately after the reservoir of fluid product has been filled or topped up and tends to decrease with the passage of time. Unless the delivery circuit is calibrated continuously and repeatedly, it is extremely difficult, if not impossible, to obtain good precision and repeatability of delivery over time of the delivery circuit of the prior art.
Another solution, adopted in circuits of a more traditional design for dispensing colouring agents, consists in providing a controlled delivery valve, that is to say, a valve disposed on the dispensing duct, preferably disposed in the vicinity of the delivery nozzle for the fluid product, which opens on command for the time necessary to deliver the desired quantity thereof. Apart from the fact that the controlled or actuated valves of the above-mentioned type are expensive, require constant maintenance and normally have to provide a bypass with the possibility of re-circulating the fluid product to the reservoir when the valve is in the closed position, with a consequent complication of the dispensing circuit and the control thereof, it should be noted that the actual quantity of fluid product delivered is a function both of the opening time of the actuated valve and of the flow of fluid in the delivery duct. The value of this last parameter, unless suitable sensors are provided or special circuit configurations comprising pressure-limiting valves or the like are produced, is difficult to determine owing, again, to the presence of a non-specific quantity of air which is variable over time and which is incorporated in the incompressible fluid inside the dispensing circuit and, in particular, in the pumping unit and in the dispensing duct upstream of the controlled valve.
U.S. Pat. No. 5,578,752 discloses a process and device for determining the flow rate of a substance conveyed by means of a thick matter piston pump through a delivery pipe. The feeding pressure in the delivery pipe is measured at predetermined time intervals by means of a pressure sensor. The interval between successive pressure strokes, used to determined the number of frequency of strokes, as well as the filling level of the delivery cylinder, used to determine the actual delivery volume per pressure stroke, are both derived from the time-dependent amplitude characteristic of the measured delivery pressure.
U.S. Pat. No. 4,808,092 discloses a precision dosing pump intended especially for use in High Performance Liquid Chromatography (HPLC) technology, wherein the pump-head housing, the pump-head bush and the cartridge-like valve units are made of a transparent material. The movement of the balls in the valve units can be detected by means of an optical sensor system in order to obtain a control signal for a control circuit for compensating the effect of the specific compressibility of the delivery medium.
International patent application WO-A-91/08445 discloses a flowmetering apparatus wherein the flow of a multi-phase fluid and of the gas and liquid fractions it contains is measured in a piston pump by sensing the position of a fluid compressive piston as by a pressure transducer or by sensing opening of a discharge valve opening at a known pressure.